In recent years, gallium nitride (GaN)-based electronic device has been considered as a promising high breakdown voltage/high speed device because of its physical characteristic. Researches on its manufacturing technique, application technique, and the like have been in progress. Generally, in order to improve a high-speed characteristic of a GaN-based electronic device, adopting a recess structure as a gate as shown in FIG. 2 is thought to be effective.
To form the recess structure, a GaN channel layer 102 and an AlGaN carrier supply layer 103 are sequentially formed on a substrate 101, and an n-type GaN layer 104 is further formed on the entire surface, and thereafter is dry-etched. Thereafter, a source electrode 106a, a drain electrode 106b, a gate electrode 108, and a SiN film 109 are formed. However, the n-type GaN layer 104, the AlGaN carrier supply layer 103, the GaN channel layer 102, and so on are easily damaged by the dry etching.
A possible method for avoiding such damage is wet etching. For example, Appl. Phys. Lett. 71 (1977) 2151-2153, Appl. Phys. Lett. 72 (1998) 560-562, J. Appl. Phys. 89 (2001) 4142-4149, Appl. Phys. Lett. 84 (2004) 3759-3761 disclose methods of wet-etching a GaN layer.
In the methods described in Appl. Phys. Lett. 71 (1977) 2151-2153, and Appl. Phys. Lett. 72 (1998) 560-562, a compound semiconductor wafer on which ohmic electrodes 106 are formed is first fabricated as shown in FIG. 3A. Then, as shown in FIG. 3B, the compound semiconductor wafer is immersed in a potassium hydroxide (KOH) aqueous solution 112 in a tank 111. Then, a Pt electrode 113 is put in the KOH aqueous solution 112 and a bias is applied, with a part of the ohmic electrodes 106 serving as an anode and the Pt electrode 113 serving as a cathode. Further, an n-type GaN layer 104 is irradiated with an ultraviolet illumination (UV).
As a result, electron-hole pairs are generated on a surface of the n-type GaN layer 104 as shown in FIG. 4A. Then, the electrons move toward the Pt electrode 113 via the ohmic electrodes 106, and the holes bind with OH− ions in the KOH aqueous solution 112 on the surface of the n-type GaN layer 104. In this manner, the oxidation and dissolution of the surface of the n-type GaN layer 104 are repeated, so that the wet etching progresses.
However, in such wet etching using the ultraviolet illumination, it is important to move the electrons from the surface of the n-type GaN layer 104 with preventing the electrons excited by the ultraviolet illumination from recombination with the holes. Further, in order to reduce variation among electronic devices, uniform processing in a wafer is also important. In the above-described method, however, a resistance present in the n-type GaN layer 104 causes variation in strength of an electric field depending on a distance from the points to which the bias is applied. Consequently, there occurs a difference in velocity of the electrons, which is likely to cause variation in etching depth in the wafer. Further, when element isolation regions 105 exist as shown in FIG. 3B, there exist regions to which no bias is applied. These regions are not etched since the electrons do not move therein as shown in FIG. 4B. Further, in the GaN-based compound semiconductor device, aluminum (Al) electrodes are generally used as the ohmic electrodes, and there is a problem that Al is easily corroded by the KOH aqueous solution.
In the methods described in J. Appl. Phys. 89 (2001) 4142-4149, and Appl. Phys. Lett. 84 (2004) 3759-3761, as shown in FIG. 5, a compound semiconductor wafer is immersed in a mixed solution 122 of a KOH aqueous solution and a potassium peroxodisulfate (K2S2O8) aqueous solution, and the n-type GaN layer 104 is irradiated with an ultraviolet illumination. This method includes no bias application. In this method, the etching of the n-type GaN layer 104 progresses since electrons excited by the irradiation of the ultraviolet illumination move toward photodissociated sulfate active ions SO4−* of peroxodisulfate ions S2O82−. Further, in this method, a mask 121 made of metal such as platinum or titanium is used as an etching mask. This is intended to make it easy for the electrons induced by the irradiation of the ultraviolet illumination to flow into the sulfate active ions in the etching solution via the metal mask 121.
However, in this conventional method, the easiness with which the electrons flow into the sulfate active ions is not uniform. Specifically, since this flow of the electrons easily occurs in the vicinity of an end surface of the metal mask 121, the etching rate in this area is higher than the etching rate in an area distant from the metal mask 121. As a result, the etching rate becomes non-uniform, resulting in uneven etching as shown in FIG. 5.
Further, when the metal mask 121 is used, prior to the formation of the ohmic electrodes, the metal mask 121 has to be removed after the wet etching. However, the removal of the metal mask 121 is very difficult. Therefore, an etched surface is sometimes contaminated with the metal. In this case, a yield is lowered.
Using a SiO2 film, which is an insulation film, as the etching mask is also described as an example, but this also lowers the yield similarly to the use of the metal mask 121 since the processing and removal of the SiO2 film are complicated.
Further, the KOH aqueous solution and the K2S2O8 aqueous solution both have high concentration (about 0.1 mol/liter). Therefore, morphology of the etched surface is low. Further, in another effort to use the ohmic electrodes as the metal mask 121, a resist mask itself necessary to protect the ohmic electrodes cannot resist the mixed solution 122. Therefore, the ohmic electrodes cannot be formed before the wet etching.
As described above, it has been conventionally difficult to process an n-type GaN layer by wet etching.